1. Field of the Invention
The present invention relates to a thermoplastic resin composition. Particularly, the present invention relates to a thermoplastic resin composition comprising a polypropylene and an olefin copolymer rubber. This composition has superior mechanical properties such as flexural strength, thermal rigidity resistance and impact resistance. The composition is especially suitable as a material for preparing home electric parts, etc.
2. Discussion of the Related Art
In recent years, a crystalline propylene-ethylene block copolymer has been used as a forming material for such applications as, for example, home electric parts. This crystalline propylene-ethylene block copolymer exhibits a balance among such properties as flexural modulus, heat distortion temperature and impact strength; however, when used alone the block copolymer has a drawback insofar as it exhibits insufficient impact strength at low temperature.
Japanese Patent Publication (Unexamined) Nos. Sho 53-22552 (1983) and Sho 53-40045 (1983) disclose incorporating an ethylene-propylene copolymer rubber into the crystalline propylene-ethylene block copolymer for improving the low-temperature impact strength of the crystalline propylene-ethylene block copolymer. This composition, however, is inferior in flexural modulus and thermal properties such as heat distortion temperature and the like because of the addition of the ethylene-propylene copolymer rubber. To overcome this problem, it is disclosed in, for example, Japanese Patent Publication (Unexamined) Nos. Sho 51-136735 (1976) and Sho 53-64256 (1978), Sho 53-64257 (1978), Sho 57-55952 (1982), Sho 57-207630 (1982), Sho 58-17139 (1983), Sho 58-111846 (1983), and Sho 59-98157 (1984), and Japanese Patent Publication (Examined) No. Sho 55-3374(1980) and the like, to add an inorganic filler such as calcium carbonate, barium sulfate, mica, crystalline calcium silicate, talc and the like.
Further, recently, in Japanese Patent Publication (Unexamined) Nos. Hei 5-59251 (1993) and 5-230321 (1993), there is proposed a resin composition improved in flexural modulus, surface hardness and the like by varying the isotactic pentad fraction of propylene homopolymer portion in an ethylene-propylene block copolymer. In Japanese Patent Publication (Examined) Nos. Sho 36-11240 (1961), Sho 38-3126 (1963), Sho 41-21785 (1966) and the like, it is proposed that impact resistance is improved by adding a vulcanized rubber to olefinic plastics. However, blending of the vulcanized rubber reduces flowability, thereby impairing the appearance of the cured composition. To overcome this problem and improve flowability, there is proposed in Japanese Patent Publication (Examined) Nos. Sho 56-15740 (1981) and 56-15743 (1981) and the like, a composition prepared by blending a mineral oil type softening agent and a peroxide non-crosslinking type rubbery material. Further, in Japanese Patent Publication (Unexamined) No. Hei 6-145437 (1994), there is proposed a material having a good balance between impact strength and rigidity.
Recently, there has been an increasing demand for materials for preparing home electronic parts that exhibit increased thermal rigidity resistance. However, conventional materials do not exhibit improved thermal rigidity resistance without sacrificing low-temperature impact resistance, since low-temperature impact resistance and thermal rigidity resistance are antipodal properties to each other.